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Measurement of Particle Size Distribution in Portland Cement Powder: Analysis of ASTM Round Robin Studies

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Chiara F. Ferraris Building and Fire Research Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899 USA Vincent Hackley Materials Science and Engineering Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899 USA and Ana Ivelisse Avilés Information Technology Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899 USA

Reprinted from Cement, Concrete and Aggregates, Vol. 26, No. 2, 2004.

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This paper is a contribution of the National Institute of Standards and Technology and is not subject to copyright.

Cement, Concrete, and Aggregates, Dec. 2004, Vol. 26, No. 2 Paper ID CCA11920 Available online at: www.astm.org

Chiara F. Ferraris,1 Vincent A. Hackley,2 and Ana Ivelisse Avil´es3

Measurement of Particle Size Distribution in Portland Cement Powder: Analysis of ASTM Round Robin Studies

ABSTRACT: A distribution of particle sizes or particle size distribution (PSD) is a fundamental characteristic of cement powder. Accurate PSDs are required in computational efforts to model the hydration process and it is an important practical issue for the cement industry. Presently, the only available standard method for measuring the PSD of cement, namely ASTM C115, is limited in scope, with a lower size detection limit of 7.5 µm. Since there are no standard procedures that adequately cover the broad particle size range associated with portland cement powder, the implementation of different measurement techniques varies widely within the industry. Two ASTM-sponsored round robin tests were performed to (1) ascertain the techniques and methods currently used in the cement industry and (2) develop and refine a standard method or methods. The results have been incorporated into a best practice method based on the technique of laser diffraction. The aim of the current paper is to summarize the findings based on the data generated during the round robins and to summarize the various approaches available to measure the PSD of cement. A summary of the statistical analysis of the test results is described. KEYWORDS: cement, particle size distribution, PSD round robin, laser diffraction, reference material

Introduction Accurate measurement of the particle size distribution (PSD) of a cement powder is both an important practical issue for the cement industry and a key limiting factor in on-going computational efforts to simulate the microstructure and predict the performance of cement-based materials (Bentz et al., 1999a, 1999b; Garboczi et al., 2003). Hence, the PSD is essential for the complete characterization of a cement powder and is closely linked to cement performance. Presently, the only relevant standard method is ASTM C115-96, a turbidimetric method for determining fineness (Standard Test Method for Fineness of Portland Cement by the Turbidimeter ASTM C115). This method is limited in scope, however, with a lower size detection limit of 7.5 µm. Because there are no standard procedures that adequately cover the broad particle size range associated with portland cement powder, the implementation of different measurement techniques varies widely within the industry. Cement is a problematic material with respect to the application of PSD methods. First, the size distribution itself is extremely broad, typically spanning two or three decades from the submicrometer range to 100 µm. In general, sizing techniques work best over a limited size range. The optimum range for a particular technique varies according to a number of factors, including detector sensitivity, underlying principle of measurement, and assumptions

Manuscript received April 15, 2003; accepted for publication May 1, 2004; published December 2004. 1 Physicist, National Institute of Standards and Technology, 100 Bureau Dr., Gaithersburg MD. 2 Research Chemist, National Institute of Standards and Technology, 100 Bureau Dr., Gaithersburg MD. 3 Mathematical Statistician, National Institute of Standards and Technology, 100 Bureau Dr., Gaithersburg MD.

inherent in the data analysis routine. Secondly, cement particles are highly agglomerated in the dry state, and therefore must be dispersed in order to differentiate between weakly bound agglomerates and primary units. Standard protocols for dispersing cement particles before analysis are nonexistent. The degree of dispersion achieved in dry aerosol methods will likely vary depending on particle size, the geometry of the dispersing device, the residence time in the sensing zone, and the applied shear force. Similarly, wet dispersion methods are subject to variations in surface chemistry of the powders, solids concentration, the nature of the medium, and the amount of mechanical energy expended to break up agglomerates. In addition, the most dispersive medium for cement, namely water, cannot be used due to the reactive nature of the solid phase. Dispersion introduces a potentially large source of variation at the sample preparation stage. Finally, cement particles are inhomogeneous in composition and often irregular (non-spherical) in morphology. Applicable commercial techniques are designed specifically for, or work best with, homogeneous spheres. Typically, an effective spherical particle diameter is reported. The degree to which irregularity affects the results will vary by technique, and is generally not well understood or properly accounted for in most methods (Bowen, 2002; Barth and Sun, 1985; Beddow and Meloy, 1980). The issue of how to determine if a chosen method of analysis yields the true distribution must also be addressed. Within this context, defining a “true” PSD is an integral part of the method development and validation process. Currently, no material artifact (i.e., reference material) or universally accepted measurement method exists for cement powder PSD determination. Therefore, a PSD reference material for cement should be established, but also a standardized method for dispersing and analyzing test samples must be coupled to this reference. To address these issues, two round robin tests, sponsored by ASTM Task Group C01.25.01, were conducted. The initial round

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Copyright by ASTM Int'l (all rights reserved); Reproduction authorized per License Agreement with CHIARA F FERRARIS (NIST - BFRL); Tue Nov 30 11:31:29 EST 2004

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2 CEMENT, CONCRETE, AND AGGREGATES robin involved 21 participants and was nonspecific with regards to methodology (i.e., participants used their in-house methods, and no protocols or parameters were specified). Four portland cements provided by the Cement and Concrete Reference Laboratory (CCRL) were included in the test: 131, 132, 135, and 136 (numbers were assigned by CCRL). The compositions and characteristics of these cements, measured as part of the CCRL proficiency program, are provided elsewhere (Ferraris et al., 2002a). The cement powder standard reference material (SRM) 114p was also included in this test. SRM 114p is routinely used to calibrate Blaine as well as other surface area measurements. The information requested from the participants included only the measurement technique and the cumulative PSD, so a detailed understanding of the various procedures used was not possible. The principal purpose of this initial round robin was to assess the overall variability in PSD measurements across the cement industry. A total of four techniques were reported by this group of participants: laser diffraction (LAS), electrical zone sensing (EZS), X-ray gravitational sedimentation (XRS), and scanning electron microscopy (SEM). Laser diffraction, using either wet or dry dispersion methods, was by far the most frequently reported technique. The high degree of variability in the data reported from the first round robin indicated the necessity for further research and testing. The second round robin expanded to 41 participants and included both specified and nonspecified measurement methodologies. Two portland cements provided by CCRL were included in the tests: 143 and 144. The characteristics of these cements, as measured in the CCRL proficiency program, are given elsewhere [Ferraris et al., 2002b]. SRM 114p was again included, with the specific intent of establishing a consensus reference PSD. Establishing a true analytical PSD for SRM 114p was considered to be neither practical nor fundamentally sound, given our present limited understanding of cement particle dispersion and the limitations of available analytic methods. In the second test, only three techniques were reported (LAS, EZS, and SEM), with 93% of participants using LAS. The issue of how to best disperse cement powder for PSD analysis was addressed by conducting additional studies at the National Institute of Standards and Technology (NIST), which were then incorporated into the design of the second round robin. The purpose of this paper is then to summarize findings based on the data generated during the round robins and to summarize the various approaches available to measure the PSD of portland cement. A summary of the statistical analysis of the test results is described. The analysis of reported data is conducted in two parts. In the first part, an attempt is made to establish a consensus reference PSD based on SRM 114p. This is followed by an examination of the parameters and methodology used by the participants in order to initiate discussion on developing a standard test method for cement PSD to be submitted for ASTM consideration. The complete set of raw data collected during the round robin tests, and the accompanying statistical analyses, are available in two separate reports (Ferraris et al., 2002a, 2002b). Description of Techniques4 Although a wide variety of techniques are available for the determination of PSD in powders, only a relative few appear to be 4 Commercial equipment, instruments, and materials mentioned in this report are identified to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology (NIST), nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

used currently within the cement industry. The choice of technique is probably based on both capability (relative to the material properties of cement) and speed and convenience of analysis. Based on the results of the two ASTM round robin studies, the currently utilized measurement techniques are: laser diffraction, electrical zone sensing (Coulter principle), sedimentation (X-ray sensing), scanning electron microscopy (imaging). Sieving, though known to be used by the cement industry in the past, was not reported as a technique of choice by any of the round robin participants. The technique most often reported was laser diffraction. In fact, only a handful of participants reported using techniques other than laser diffraction (less than 10%). In this section, we provide a synopsis for each technique as it relates to cement analysis. Greater detail is provided for laser diffraction, since it is the most widely used method for cement powder sizing.

Laser Diffraction (LAS) In the LAS technique, the angular distribution of light scattered from a dilute particle dispersion is measured. To be precise, light can be scattered, diffracted, or absorbed by the dispersed particles (Bohren et al., 1983). Scattered light consists of reflected and refracted waves and depends on the form, size, and composition of the particles. Diffracted light arises from edge phenomena and is dependent only on the geometric shadow created by each particle: diffraction is independent of the composition of the particles. Absorption occurs when light is converted to heat or electrical energy by interaction with the particles, and is influenced by both size and composition. The so-called laser diffraction technique incorporates all three of these effects, but is generally limited to the more forward scattering angles. The key material parameter for LAS is the complex refractive index, m = n − ik, where n is the real component and k is the imaginary (absorptive) component. Scattering arises due to differences in the refractive index of the particle and the surrounding medium (and internal variations in the case of heterogeneous particles). Values of n have been published for many bulk materials (see for example Handbook of Optical Constants of Solids, 1985, 1991), but in the case of cement, n is routinely estimated based on a mass average of the refractive indices for the individual material components (Cyr et al., 2000). Absorption becomes important primarily in the fine fraction, especially below 1 µm. Cement is generally gray to off-white in color, and therefore a finite, but relatively low value for the imaginary component is expected. The value k = 0.1 is often reported for cement, although the origin of this value is unclear and its appropriateness for general use has not been established. There are two principal methods of data analysis for LAS: Mie and Fraunhofer. Mie theory describes scattering by homogeneous spheres of arbitrary size and is the most rigorous optical scattering model available. For non-spherical particles, Mie provides a volume-weighted equivalent spherical diameter. Mie theory has been applied with mixed success to the analysis of powders with diameters from several hundreds of micrometers down to about 100 nm. An accurate representation of the “true” size distribution by Mie scattering is dependent on the input of an accurate value for the complex refractive index. For particles much larger than the wavelength of light, the Fraunhofer method can be used without knowledge of the refractive index, because it is based on the diffraction effect only. The range of validity for Fraunhofer is limited at the fine end to diameters a few times greater than the


FERRARIS ET AL. ON ASTM ROUND ROBIN STUDIES

wavelength of light, denoted by λ, for particles that are opaque or have a large refractive index contrast with the medium (ISO 133201:1999(E)). For more transparent particles, or particles with a moderate refraction contrast, the lower limit is raised to about 40 × λ. For λ = 633 nm (red light), this corresponds to about 25 µm. The benefit of using Fraunhofer diffraction is that the interpretation is not dependent on the absorptive or refractive properties of the material. On the other hand, use of the Fraunhofer approximation beyond the valid range can lead to large systematic errors in the calculated PSD (ISO 13320-1:1999(E)). The LAS method requires that the particles be in a dispersed state, either in liquid (suspension) or in air (aerosol). The former is presently referred to as the “wet” method (LAS-W) whereas the latter is termed the “dry” method (LAS-D). Differences between LAS-D and LAS-W methods arise primarily from the different ways in which the particles are dispersed in each case. In liquid, it is possible to modify solution conditions by changing pH or adding chemical dispersing agents, and one can disrupt aggregates using mechanical or ultrasonic energy. Thus, for the very fine fraction, a better state of dispersion can be achieved in an appropriately selected liquid medium. Generally, water is an excellent dispersing medium. However, due to the reactive nature of cement in water, alcohols, such as isopropanol, methanol, and ethanol, are commonly used in its place. In the LAS-D method, a stream of compressed air (or a vacuum) is used to both disperse the particles and to transport them to the sensing zone. This method of dispersion works best for the coarse size fraction, where the interparticle contacts are weak. For particles smaller than a micrometer, or highly asymmetric particles, air dispersion is generally not appropriate for sizing. Electrical Zone Sensing (EZS) Electrical Zone Sensing is based on the Coulter principle. The powder is dispersed at a highly dilute concentration in a conducting liquid, which is then drawn through a small orifice in an insulating wall on either side of which are placed electrodes. As a particle enters the orifice, or sensing zone, the volume of solution displaced by the particle causes a transient change in the measured electrical impedance across the opening. The amplitude of the impedance pulse is proportional to the particle’s volume. By accumulating pulses over time, a PSD is constructed. EZS is a particle counting method capable of producing a number-weighted or mass-weighted distribution of particle sizes and requires calibration. Different size orifices are used to capture broad PSDs. The applicable particle size range is from about 0.2–800 µm, although a lower limit of 0.6 µm is probably more realistic for normal operating conditions (Allen, 1990; ISO 13319:2000(E)). Errors can arise from coincident passage of multiple particles through the orifice and from high asperity particles, both of which skew the PSD towards larger sizes. A pulse discrimination system can be used to correct for the coincidence effect by rejecting distorted pulses. Porous particles (e.g., fly ash) are generally unsuitable for EZS measurements because their effective densities are not known. The dispersion procedure and solids concentration are critical parameters for accuracy in EZS measurements. Sedimentation (XRS) The change in concentration or density of a moderately dilute (1–6% mass fraction) suspension with time is measured at known depths, using optical or X-ray sensing. X-ray gravitational sedimen-

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tation (XRS) is well established in several industries. Sedimentation methods are based on the application of Stokes’ Law, which describes the terminal velocity for an isolated sphere settling in a viscous fluid under the influence of a gravitational field. Stokes’ Law is valid only if the Reynolds number (Re) does not exceed about 0.25 (ISO 13317-1:2001). Using a density value of 3.2 g/cm3 for portland cement, the largest (equivalent spherical) diameter that can be sized accurately with XRS is about 95 µm in isopropanol (IPA) at 25 ◦ C. Particles larger than this will settle much more slowly than predicted by Stokes’ Law. The upper size limit can be increased by using a higher viscosity fluid. Irregularly shaped particles should settle according to their equivalent spherical volume at low Re values. For fines, the effect of Brownian motion exerts a significant influence on settling at diameters below about 1 µm in water and about 0.7 µm in IPA. Convection currents in the settling suspension may further limit the lower size range. Parameters required for XRS are solid and liquid phase density, and liquid phase viscosity. The particles must remain stable against agglomeration during settling or the measured PSD will be skewed toward larger sizes. Scanning Electron Microscopy (SEM) The SEM is an analytical tool that uses a focused beam of electrons to form magnified images. Under ideal conditions, the SEM is capable of producing images with a feature resolution at the nanometer level. In addition to image capabilities, a properly equipped SEM can provide information on the elemental composition of microscopic features. Information on the image and elemental characteristics of a sample are obtained through the interaction of the electron beam with the sample material, which produces various effects that can be monitored with suitable detectors. The resulting signals, which include secondary and backscattered electrons along with characteristic photoelectron X-rays, can be collected in synchronization with the position of the electron beam to provide detailed spatial and compositional information. A computer controlled SEM (CCSEM) can provide simultaneous measurement of individual particle size, shape (aspect ratio), and elemental composition by combining an SEM, an X-ray analyzer (EDS), and a digital scan generator under computer control [Schwoeble et al., 1988]. Use of the computer to control the analysis permits relatively large numbers of individual particles to be analyzed (e.g., 1000 s). Elemental composition, though an important benefit for research purposes, was not part of the round robin tests. Errors typically arise because of poor counting statistics for large particles, an inability to differentiate between primary particles and agglomerates, 3-D/2-D effects, and/or artifacts created during sample preparation.

Data Analysis Methodology In both round robins, the results for SRM 114p were analyzed separately from the other cements with the objective of producing a reference curve that instrument operators could use to “calibrate” their systems or to validate their methodology. In other words, the reference distribution of SRM 114p could potentially be used to check that the PSD obtained by a particular instrument falls within a defined margin of error, or it could be used to offset measured values by a size-range-dependent factor to bring them within the acceptable margin of error. The more significant errors associated with the measured PSD curves coming from different laboratories and users are most likely due to systematic differences that result because devices differ in systematic ways on nuisance factors. To


4 CEMENT, CONCRETE, AND AGGREGATES determine the reference distribution for SRM 114p, two approaches were considered:

r Approach 1: Establish a single calibration curve that represents an average distribution for all techniques inclusive (i.e., all-inclusive approach); r Approach 2: Establish a calibration curve for each technique (i.e., technique-specific approach) Both approaches have advantages and disadvantages. In the first approach (all-inclusive), the calibration curve would be less precise (greater margin of error) as a result of cumulative variations in the precision of different methods. On the other hand, the first approach is simple and convenient, because every customer would use the same calibration curve. In the second approach (technique-specific), the calibration should be more precise, because variations resulting from differences in measurement principle or precision would be eliminated. As a disadvantage, several calibration curves would need to be established independently, one curve for each method, and this would require a statistically relevant pool of round robin participants for each technique. The best possible approach, of course, would be to establish a calibration curve based on an independent determination of the “true” PSD, using a method that can be validated theoretically and/or experimentally; such a method does not currently exist for cement. In the second round robin (used as the basis for development of a reference curve), 39 participants (93% of all participants) used the LAS technique. Of these, 26 (62% of all participants) dispersed their powders in a liquid (LAS-W) and 13 (31%) used a dry powder method (LAS-D). On the other hand, there was only one participant that reported using SEM and two who reported using EZS. Therefore, using the technique-specific approach (Approach 2), a statistically relevant calibration curve could only be determined for LAS-W and LAS-D. Obviously, all 42 sets could be used if the all-inclusive method (Approach 1) was followed, but the resulting curve would be heavily weighted by the relatively large number of LAS results. Therefore, it was determined that an all-inclusive calibration curve was not appropriate for this database and the development of a reference PSD curve was limited to the LAS technique. To determine the curve that best represents the consensus PSD for LAS, outliers must first be identified and excluded from calculation of the reference curve. The method that was adopted here is based on calculation of the mean and the two-sided 95% confidence limits using the bootstrap method. The bootstrap method replaces difficult (or even impossible analytical) solutions to statistical problems with raw computing power (details on the bootstrap method are provided in Appendix C of ref. [Ferraris et al., 2002a]). The bootstrap method does not explicitly provide the criteria needed to determine outliers. Therefore, we selected the following criteria for elimination of outliers: if more than 27% of points (i.e., four data points) in a single round robin data set (i.e., a curve provided by one round robin participant) exceed by more than 5% the confidence limits determined by analysis of all data sets in the grouping (i.e., curves provided by all participants), then this individual data set is identified as an outlier. The tolerance value of 5% is based on the absolute difference between the measured value and the closest confidence limit value. Once the outliers are determined, the mean and 95% confidence limits are recalculated excluding the outliers. The resulting mean curve is then defined as the consensus reference curve representing SRM 114p. No judgment was made on how the

data were generated in the LAS, i.e., what parameters were used such as refractive indices, dispersion methodology, etc. Discussion of these parameters will be given below. Determination of the Reference PSD for Laser Diffraction For LAS measurements, both wet and dry, two sets of results were collected:

r Participant’s in-house method (PM): the participants were requested to use the method that they normally use and to describe it in detail. r Specified method (SM): all participants were requested to use a set of parameters specified by NIST (Appendix D of ref. [Ferraris et al. 2002b]). We have examined the two sets of data separately and then in combination, after first excluding the outliers. As a result of this process, three mean bootstrap curves with 95% confidence limits are provided for both LAS-W and LAS-D. The issue is then to decide which mean curve or curves is most appropriate as a reference for SRM 114p using the LAS technique. Note that high and low intervals are not symmetric about the mean. These limits can be asymmetric around the sample mean. This is caused by a slight skewness of the generated bootstrapped sampling distribution of the mean. All individual data sets used in this analysis are reported elsewhere [Ferraris et al., 2002b]. Six outliers were found for LAS-W out of 25 participants, and two outliers were determined for LAS-D out of 13 participants. The bootstrap results are given in Table 1 for LAS-W and Table 2 for LAS-D. A graphical comparison of the three distributions is provided in Fig. 1 for LAS-W and Fig. 2 for LAS-D. As shown in these figures, the differences between the three PSDs are not very large. Because the combined distribution represents a larger statistical pool, it could be argued that the combined PSD should be used as a single consensus reference curve. Correction Procedure for Unknown PSDs The purpose of a reference PSD based on an easily accessible reference material is twofold: to verify the efficacy of the instrument or method being used, and to correct measurement results by applying a set of correction factors. A procedure in which the mean PSD curve is used to correct measured data obtained using various instruments would work in the following manner: 1) Calculate the correction factor for each size, defined as the ratio between the measured value and the mean value as shown in Table 1 for LAS-W and Table 2 for LAS-D; 2) Multiply measured size data for unknown samples by this correction factor. This procedure was applied to all available data sets for CCRL 143 and 144, which were provided to the participants of the second round robin study. Figure 3 shows some selected results from this analysis. The complete results are provided in a separate publication [Ferraris et al., 2002b]. It was initially expected by the ASTM committee that a single method and reference PSD could be used to correct all measurements; however, in practice this proved problematic. If the target measurement results themselves (not the calibration curve) contain outliers, (i.e., data points that are more than 5% outside the confidence limits obtained with the bootstrap method), the correction is not sufficient to bring the entire curve within the confidence limits of the calibration curve



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TABLE 1—Bootstrap data for LAS-W (after excluding outliers). Size refers to equivalent spherical particle diameter.

PM Only Size [µm] 1 1.5 2 3 4 6 8 12 16 24 32 48 64 96 128

PM + SM

SM Only

Mean

Low

High

Mean

Low

High

Mean

Low

High

5.0 7.9 11.1 16.3 20.5 27.9 34.1 45.8 54.7 69.7 80.4 92.6 97.1 99.4 99.9

3.5 6.4 9.6 14.8 19.0 26.5 32.8 43.9 52.7 68.0 78.8 91.5 96.4 99.0 99.7

6.3 9.5 12.5 17.8 22.1 29.4 35.5 47.8 56.6 71.6 82.1 93.7 97.9 99.7 100.0

6.0 9.5 13.7 19.0 23.4 30.7 37.3 49.0 58.0 72.5 83.4 94.3 98.3 99.7 100.0

3.9 7.2 11.6 16.7 21.1 28.3 34.9 45.7 55.0 69.8 81.3 93.0 97.5 99.5 99.9

8.3 12.1 15.9 21.3 25.7 33.3 39.7 52.1 61.2 75.1 85.3 95.5 98.9 99.9 100.0

5.4 8.7 12.2 17.5 21.8 29.1 35.5 47.2 56.2 71.0 81.7 93.4 97.6 99.6 99.9

4.2 7.2 10.9 16.2 20.4 27.7 34.1 45.4 54.2 69.5 80.3 92.5 97.1 99.3 99.8

6.7 10.0 13.5 19.0 23.2 30.6 37.0 49.1 57.9 72.6 83.1 94.2 98.1 99.8 100.0

TABLE 2—Bootstrap data for LAS-D for PM (after excluding outliers). Size refers to equivalent spherical particle diameter.

PM Only

PM + SM

SM Only

Size [µm]

Mean

Low

High

Mean

Low

High

Mean

Low

High

1 1.5 2 3 4 6 8 12 16 24 32 48 64 96 128

5.0 7.9 12.3 18.1 23.0 30.9 37.1 47.3 55.3 68.9 78.8 90.7 95.9 98.7 99.1

2.7 4.5 10.2 15.5 20.2 27.6 33.9 44.0 52.2 65.9 76.4 88.7 94.2 97.5 97.9

7.3 10.9 14.5 20.6 26.0 34.4 40.5 50.7 58.4 71.6 81.2 92.6 97.5 99.7 99.9

4.4 6.7 11.9 17.2 21.5 28.7 34.7 45.3 53.7 67.9 78.6 91.5 96.7 99.5 99.9

2.6 3.9 10.3 15.4 19.3 26.0 32.1 43.2 52.0 66.7 77.6 90.4 95.9 99.2 99.8

6.2 9.2 13.3 19.3 23.6 30.8 36.7 47.3 55.4 69.3 79.7 92.7 97.7 99.8 100.0

4.7 7.3 12.1 17.7 22.3 29.9 35.9 46.4 54.6 68.4 78.7 91.1 96.3 99.1 99.5

3.3 5.4 10.6 16.1 20.3 27.8 33.9 44.4 52.8 66.9 77.3 89.9 95.3 98.3 98.8

6.1 9.5 13.4 19.3 24.2 32.0 38.1 48.4 56.7 70.1 80.2 92.2 97.3 99.6 100.0

FIG. 1—Graphical comparison of the Bootstrap mean curves obtained in Table 1 (LAS-W). For clarity, the confidence intervals are not shown here, but can be found in Table 1.

FIG. 2—Graphical comparison of the Bootstrap mean curves obtained in Table 2 (LAS-D). For clarity, the confidence intervals are not shown here, but can be found in Table 2.


6 CEMENT, CONCRETE, AND AGGREGATES their in-house methods. This information was kept confidential, such that no specific technique, method or set of results could be identified with a particular organization. In this section, we will examine the methods used and determine if it is possible to develop a “best practice” that could eventually be presented to ASTM for approval as part of a standard test method. Only the information submitted for laser diffraction will be examined here, as we do not have statistical representation for other methods. Laser Diffraction with the Specimen Dispersed in a Liquid (LAS-W) Participant-provided information concerning in-house methodology was divided into two areas: sample preparation and analysis. It is important to examine the responses with two goals in mind: 1) can a consensus procedure or procedures be established, and 2) can we identify parameters that significantly impact results? Summary of Industry Practice for Sample Preparation

FIG. 3—Corrections of selected data for cement CCRL 143. A) Data as measured; B) Data corrected. In the legend, 450, 611, and V6 represent coded data sets from participants in the round-robin. Also shown in these graphs are the 95% limits.

(Fig. 3). On the other hand, if the data set lies completely within the confidence limits defined by the reference curve, the correction factor will reduce the overall spread of the data. Therefore, the reference curves for SRM 114p could be used in two ways: 1) As part of a validation method, to check that measurements are within the confidence limit range of the reference. This will allow the operator to determine if sample preparation problems or possibly a malfunctioning instrument should be considered; 2) As a calibration curve to correct data from unknown samples, after the method has first been validated as mentioned above. This correction should be used carefully, since the corrected PSD could remain outside the acceptable range. Measurement Methodology The scope of this study was also to compare the test sample preparation and measurement parameters used within the cement industry for a given technique. To facilitate this comparison, participants were asked to provide specific detailed information about

In the area of sample preparation, the following key information was requested where appropriate: dispersion medium, solids concentration, dispersion procedure, surfactant (if used), and type and duration of ultrasonic treatment (if used). Each of these items should be clearly defined if a standard test is proposed to ASTM. Only a summary will be given here as the details are described in reference [Ferraris et al., 2002b]. Most LAS-W participants used alcohol as a dispersion medium: 54% used isopropanol (IPA), 31% used ethanol and 8% used methanol (percentage were rounded). However, the use of alcohol was not universal. Two participants (8%) used an aqueous medium, despite the obvious problem with hydration. The second issue concerns the concentration of cement in the measuring cell, and the dispersion and/or dilution method used to achieve that concentration. This information is paramount because it can affect the ability to fully disperse the cement, and could lead to a user bias or increased variability in the measurements. Furthermore, solids concentration is a key parameter for any light scattering technique, due to the effects of multiple scattering when concentrations are too high and minimum signal requirements when they are low. The majority of round robin participants prepared their cement powder suspensions in a single step (i.e., they analyzed the samples as prepared, without further dilution). In some cases, a known amount of cement was added, while in other cases the addition amount was varied to achieve a certain optical obscuration level in the cell. The optimum percentage obscuration range is predetermined by the measurement device requirements. As a result, the concentration used in the round robin varied widely and was reported explicitly by only 12 of 26 participants who used liquid dispersion. It could be concluded from these results that the most common practice is to adjust concentration in situ (i.e., with the suspending liquid in the measurement cell) based on obscuration levels. It might be difficult, therefore, to prescribe a fixed-solids concentration in a standard test method, since different instruments may require different obscuration levels. An alternative approach would be to specify the solids content for a stock concentrate, allowing some control over sample preparation, but the stock would then be diluted in situ (using the chosen suspending medium) to obtain the optimal obscuration level for a particular instrument. Another factor influencing dispersion relates to the use of ultrasonication to break up agglomerated particles, a common practice in many industries. Round robin results show that 69% of LAS-W



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FIG. 4—Correlation of reported ultrasonic treatment duration and the dispersion of the smaller size fraction represented here by D10 .

users employed ultrasonic treatment of cement dispersions before measurement. Of these, 63% used in-line ultrasonication provided by the instrument, while the remainder used an externally applied ultrasonic device prior to sample introduction to the measuring cell. One participant reported using both external and in-line ultrasonication, in series. The power and the duration of the ultrasonication should be compared to determine the best procedure for an ASTM standard. Unfortunately, the power cannot be explicitly compared because values are not always reported in fundamental units (i.e., Watts versus a relative % scale) nor is output power always clearly defined with respect to the device geometry and sample volume. Furthermore, the sound frequency is rarely known or reported. On the other hand, the duration of treatment was reported by all participants using ultrasonics, and it ranged from 10–300 s, with a median value of 60 s. A comparison of reported ultrasonic treatment durations and the measured D10 values5 is given in Fig. 4. There is no clear dependence of the fine fraction size on duration, nor is there any apparent correlation between duration and the occurrence of outliers. Further studies to determine the impact of ultrasonic treatment duration on dispersion of cement in alcoholic media were performed at NIST (Hackley et al., 2004). In these studies, an external ultrasonic immersion device was employed. Suspensions were prepared at a solids volume fraction of 5% in IPA. Results, summarized in Fig. 5, indicate that after an initial treatment duration of 60 s at an output power of 90 W, further treatment provided no additional dispersive benefit. From the resulting PSDs, it is clear that the initial treatment improved dispersion of the finer fraction (below 20 µm), while having no appreciable impact on the coarse fraction. Summary of the Analysis Methods The three specifications requested from the participants with respect to the analysis step were: duration of the measurement, model used to fit scattering results (Mie or Fraunhofer), and, if Mie, complex refractive index used (real and imaginary) for both cement and medium. The reported measurement duration varied from 4–120 s. This is a wide range that seems to depend primarily on the commercial 5 Particle diameter below which 10% of the mass of the PSD is found. Represents a characteristic particle size for the finest fraction.

FIG. 5—Cumulative PSD of cement powder (CCRL 135) in IPA as a function of ultrasonic treatment duration.

device used. Nevertheless, the majority of measurements were of 60 s duration or less, and this is clearly one reason that LAS has become so prevalent within the cement industry. However, no clear correlation was observed between measurement time and PSD results, and presumably this is because each instrument determines the length of measurement necessary to reach some internally set signal-to-noise ratio. As stated in the introduction, of the two optical models for interpreting angle-dependent scattering by particles, Fraunhofer and Mie, only the second one requires the refractive indices to be specified. According to ISO 13320-1, the Fraunhofer model works well for particle sizes >50 µm. For particle sizes